Carbon nanotube slurry, method for making the same, and method for making cathod emitter using the same

ABSTRACT

A kind of photosensitive carbon nanotube slurry is disclosed. The photosensitive carbon nanotube slurry includes a first mixture and a second mixture. The first mixture includes carbon nanotubes, conducting particles, and a first organic carrier. The second mixture includes a photo polymerization monomer, a photo initiator, and a second organic carrier. The weight percentage of the first mixture and the second mixture ranges from about 50% to about 80% and about 20% to about 50%, respectively. Methods for making the photosensitive carbon nanotube slurry and methods for making cathode emitters using the photosensitive carbon nanotube slurry are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201210058512.4, filed on Mar. 8, 2012 inthe China Intellectual Property Office, disclosure of which isincorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a carbon nanotube slurry, a method formaking the carbon nanotube slurry, and a method for making cathodeemitters using the carbon nanotube slurry.

2. Description of Related Art

Carbon nanotubes (CNT) are tube-shaped structures, and have extremelyhigh electrical conductivity, very small diameters, and a tip-surfacearea near the theoretical limit. Thus, carbon nanotubes can transmit anextremely high electrical current and can be used to make cathodeemitters.

One method for making carbon nanotubes based cathode emitters andassembling the carbon nanotubes in field emission devices isscreen-printing. The method includes steps: (a) preparing printablecarbon nanotube slurry, which usually includes carbon nanotubes, organiccarrier, glass powder and organic solvent; and (b) providing cathodeelectrodes and forming patterned cathode emitters on surfaces of thecathode electrodes by printing using a screen. The screen-printingprocess is low in cost and can be easily operated. However, the mesh ofthe screen can be blocked by particles of the carbon nanotube slurry.Thus, the cathode emitters have trouble in forming a pattern having asize less than 100 microns using the screen-printing method.

What is needed, therefore, is to provide a carbon nanotube slurry and amethod for making cathode emitters to overcome the above-describedshortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a scanning electron microscope (SEM) image of patternedcathode emitters based on carbon nanotubes of one embodiment.

FIG. 2 is a SEM image of carbon nanotubes in cathode emitters of oneembodiment.

FIG. 3 shows a current density vs. time curve of cathode emitters of oneembodiment.

FIG. 4 shows a current density vs. time curve of cathode emitters ofrelated art.

FIG. 5 is a field emission image of cathode emitters of one embodiment.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

A photosensitive carbon nanotube slurry of one embodiment includes afirst mixture and a second mixture. The first mixture includes carbonnanotubes, conducting particles, and a first organic carrier. The secondmixture includes a photo polymerization monomer, a photo initiator, anda second organic carrier. The weight percentage of the first mixture inthe photosensitive carbon nanotube slurry can range from about 50% toabout 80%. The weight percentage of the second mixture in thephotosensitive carbon nanotube slurry can range from about 20% to about50%. In one embodiment, the first mixture further includes a binder anda stabilizer.

The weight percentage of the carbon nanotubes in the first mixture canrange from about 0.1% to about 5%. The weight percentage of theconducting particles in the first mixture can range from about 20% toabout 75%. The weight percentage of the binder in the first mixture canrange from about 1% to about 25%. The weight percentage of thestabilizer in the first mixture can range from about 0.5% to about 5%.The weight percentage of the first organic carrier in the first mixturecan range from about 20% to about 70%.

In one embodiment, the weight percentage of the carbon nanotubes in thefirst mixture can range from about 0.5% to about 2%. The weightpercentage of the conducting particles in the first mixture can rangefrom about 30% to about 70%. The weight percentage of the binder in thefirst mixture can range from about 5% to about 15%. The weightpercentage of the stabilizer in the first mixture can range from about1% to about 2%. The weight percentage of the first organic carrier inthe first mixture can range from about 20% to about 55%.

The carbon nanotubes can be single-walled carbon nanotubes,double-walled carbon nanotubes, multi-walled carbon nanotubes, andcombinations thereof. The diameter of each single-walled carbon nanotubecan range from about 0.5 nanometers to about 50 nanometers. The diameterof each double-walled carbon nanotube can range from about 1 nanometerto about 50 nanometers. The diameter of each multi-walled carbonnanotube can range from about 1.5 nanometers to about 50 nanometers. Thelength of the carbon nanotubes can be in a range from about 1 micron toabout 15 microns. The carbon nanotubes can be prepared by chemical vapordeposition method, arc discharge method, laser ablation method or anyother methods.

The conducting particles can be metal particles, indium tin oxideparticles or combinations thereof. The metal particles can be goldparticles, silver particles, aluminum particles, and copper particles.The metal particles in the first mixture have high chemical stability.Therefore, the metal particles cannot be easily oxidized during aheat-treatment process and can retain a good electrical conductivity.The particle size of the conducting particles can be less than or equalto 1 micron. The specific surface area of the conducting particles canbe in a range from about 1 m²/g to about 3 m²/g.

The binder is inorganic binder in one embodiment. The binder can beglass powder, silicon oxide powder, silicone resin powder orcombinations thereof. The binder of one embodiment is a low meltingpoint glass powder with a melting point in a range from about 350° C. toabout 600° C. The effective diameter of the binder can be less than orequal to 1 micron.

The stabilizer is citric acid in one embodiment. The function of thestabilizer is to complex the ionic conducting particles and let theconducting particles evenly distribute in the first organic carrier.

The first organic carrier is a volatilizable organic material and can beremoved by heating. The first organic carrier includes ethyl cellulose,terpineol, and ethanol. The weight percentage of the ethyl cellulose inthe first organic carrier can range from about 10% to about 40%. Theweight percentage of the terpineol in the first organic carrier canrange from about 30% to about 50%. The weight percentage of the ethanolin the first organic carrier can range from about 30% to about 50%. Theethyl cellulose has strong polarity and can form a network structure orchain structure to enhance the viscosity and plasticity of thephotosensitive carbon nanotube slurry. The terpineol acts as a diluentto dissolve the ethyl cellulose and allows the photosensitive carbonnanotube slurry to have liquidity. The ethanol acts as solvent todissolve the carbon nanotubes, conducting particles, and binder.

The weight percentage of the photo polymerization monomer in the secondmixture can range from about 20% to about 60%. The weight percentage ofthe photo initiator in the second mixture can range from about 10% toabout 40%. The weight percentage of the second organic carrier in thesecond mixture can range from about 30% to about 40%.

In one embodiment, the weight percentage of the photo polymerizationmonomer in the second mixture can range from about 30% to about 50%. Theweight percentage of the photo initiator in the second mixture can rangefrom about 15% to about 35%. The weight percentage of the second organiccarrier in the second mixture can range from about 30% to about 35%.

The photo polymerization monomer can be methyl acrylate, ethyl acrylate,N-butyl acrylate, methyl methacrylate, ethyl methacrylate, N-butylmethacrylate, and combinations thereof. The function of the photopolymerization monomer is to crosslink and cure under the condition ofcertain illumination and photo initiator.

The photo initiator can be free radical photo initiator or cationicphoto initiator. The function of the photo initiator is to initiate thephoto polymerization monomer to crosslink and cure under certainillumination condition.

The second organic carrier can be terpineol or ethanol.

In one embodiment, the weight percentage of the first mixture in thephotosensitive carbon nanotube slurry is 70% and the weight percentageof the second mixture in the photosensitive carbon nanotube slurry is30%. The carbon nanotubes are multi-walled carbon nanotubes with adiameter from about 10 nanometers to about 30 nanometers and a length ina range from about 2 microns to about 5 microns. The conductingparticles are silver particles with a particle size from about 0.5microns to about 1 micron. The binder is glass powder with an effectivediameter from about 0.5 microns to about 1 micron. The stabilizer iscitric acid. The first organic carrier consists of ethyl cellulose,terpineol, and ethanol. The weight ratio of the ethyl cellulose,terpineol and ethanol is 1:2:2. The weight percentage of the carbonnanotubes in the first mixture is 0.5%. The weight percentage of thesilver particles in the first mixture is 70%. The weight percentage ofthe glass powder in the first mixture is 8.5%. The weight percentage ofthe citric acid in the first mixture is 1%. The weight percentage of thefirst organic carrier in the first mixture is 20%. The photopolymerization monomer is methyl methacrylate. The photo initiator isTPO. The second organic carrier is terpineol. The weight percentage ofthe methyl methacrylate in the second mixture is 50%. The weightpercentage of the TPO in the second mixture is 15%. The weightpercentage of the terpineol in the second mixture is 35%.

In another embodiment, the weight percentage of the first mixture in thephotosensitive carbon nanotube slurry is 80% and the weight percentageof the second mixture in the photosensitive carbon nanotube slurry is20%. The carbon nanotubes are multi-walled carbon nanotubes with adiameter from about 30 nanometers to about 50 nanometers and a length ina range from about 5 microns to about 10 microns. The conductingparticles are silver particles with a particle size from about 0.1microns to about 0.5 micron. The binder is glass powder with aneffective diameter from about 0.1 microns to about 0.5 micron. Thestabilizer is citric acid. The first organic carrier consists of ethylcellulose, terpineol and ethanol and the weight ratio of the ethylcellulose, terpineol and ethanol is 1:4:5. The weight percentage of thecarbon nanotubes in the first mixture is 2%. The weight percentage ofthe silver particles in the first mixture is 65%. The weight percentageof the glass powder in the first mixture is 10%. The weight percentageof the citric acid in the first mixture is 2%. The weight percentage ofthe first organic carrier in the first mixture is 21%. The photopolymerization monomer is methyl methacrylate. The photo initiator isTPO. The second organic carrier is terpineol. The weight percentage ofthe methyl methacrylate in the second mixture is 30%. The weightpercentage of the TPO in the second mixture is 35%. The weightpercentage of the terpineol in the second mixture is 35%.

A method for making a photosensitive carbon nanotube slurry includes thefollowing steps of:

(S1) providing carbon nanotubes, conducting particles, a binder and astabilizer;

(S2) dissolving the carbon nanotubes, conducting particles, binder andstabilizer into a first organic carrier to obtain a first mixture;

(S3) providing a photo polymerization monomer and a photo initiator;

(S4) dissolving the photo polymerization monomer and photo initiatorinto a second organic carrier to obtain a second mixture; and

(S5) mixing the first mixture and the second mixture to obtain aphotosensitive carbon nanotube slurry.

In step (S2), the carbon nanotubes, conducting particles and binder arefirstly dissolved into the first organic carrier. The stabilizer isfinally dissolved into the first organic carrier.

In step (S4), the photo polymerization monomer is firstly dissolved intothe second organic carrier. The photo initiator is secondly dissolvedinto the second organic carrier.

In step (S2), (S4) and (S5), mechanical agitation or ultrasonicagitation can be performed during the dissolving or mixing process.

In one embodiment, an optional step (S6) of mechanical extruding andshearing the photosensitive carbon nanotube slurry can be performedafter step (S5).

The step (S6) can further facilitate particles in the photosensitivecarbon nanotube slurry be evenly distributed. In one embodiment,repeated mechanical extruding and shearing of the photosensitive carbonnanotube slurry is performed. The step (S6) can be performed by rollingmachine, colloid mill, emulsifying machine or kneader. In oneembodiment, a 3-roller rolling machine is applied to treat thephotosensitive carbon nanotube slurry.

A method for making cathode emitters using the photosensitive carbonnanotube slurry includes the following steps of:

(a) coating the photosensitive carbon nanotube slurry onto a glasssubstrate;

(b) drying the photosensitive carbon nanotube slurry;

(c) exposing the photosensitive carbon nanotube slurry with a mask;

(d) developing the photosensitive carbon nanotube slurry; and

(e) heating the photosensitive carbon nanotube slurry in a vacuum or ina protection gas.

In step (a), the photosensitive carbon nanotube slurry can be coatedonto the glass substrate by spinning, spraying, dipping, rolling,brushing or screen printing. The photosensitive carbon nanotube slurryincludes carbon nanotubes, conducting particles, first organic carrier,photo polymerization monomer, photo initiator, and second organiccarrier.

In step (b), the drying temperature can range from about 50° C. to about100° C. The drying time can range from about 5 minutes to about 30minutes.

In step (c), halogen lamps, high-pressure lamps, laser or metal halidelamps can be used as exposure light source. In one embodiment, a contactexposure process is performed, so the contact of the mask and thephotosensitive carbon nanotube slurry will not destroy the structures ofthe photosensitive carbon nanotube slurry and the cathode emittersformed later. In another embodiment, a non-contact exposure process isperformed. The exposure energy can range from about 300 mJ/cm² to about500 mJ/cm².

In step (d), the developing process can be performed by spraying adeveloper onto the glass substrate or by immersing the glass substrateinto the developer. The developer used in one embodiment is sodiumcarbonate solution with a weight percentage ranges from about 0.5% toabout 1.5%.

In step (e), the function of the heating process is to melt the binderto fix the photosensitive carbon nanotube slurry onto the glasssubstrate to form cathode emitters. The heating temperature can rangefrom about 350° C. to about 650° C. The heating time can range fromabout 15 minutes to about 45 minutes. The protection gas can be selectedfrom nitrogen, argon, and helium.

In one embodiment, an optional step (f) of surface treating the cathodeemitters can be performed after step (e).

The method of surface treating can be surface polishing, plasma etching,laser etching, or adhesive tape peeling. In one embodiment, the surfaceof the cathode emitters is treated by adhesive tape to peel part of thecarbon nanotubes that is not firmly attached on the glass substrate. Theremaining carbon nanotubes are firmly attached on the glass substrate,substantially vertical and dispersed uniformly. Therefore, interferencefrom the electric fields between the carbon nanotubes is reduced and thefield emission performances of the cathode emitters are enhanced.

FIG. 1 shows patterned cathode emitters based on the photosensitivecarbon nanotube slurry made by one embodiment of the above method. FIG.2 shows structures of the carbon nanotubes in the patterned cathodeemitters of FIG. 1. It can be seen that the size of the patternedcathode emitters is smaller than 100 microns in some position. Thestability and regularity of the patterned cathode emitters are bothrelatively high.

FIG. 3 shows a field emission performances curve of the patternedcathode emitters. The emission current density of the patterned cathodeemitters continuously increases, which is significantly different fromthe trend of the cathode emitters in a related art shown in FIG. 4.Furthermore, the stable emission current density of the patternedcathode emitters (about 1.38 mA/cm²) is much higher than the stableemission current density of the cathode emitters of the related art(about 0.46 mA/cm²) after 100 minutes.

FIG. 5 shows a field emission image of the patterned cathode emitters.It can be seen that the field emission spots in the image are evenlydistributed. When the patterned cathode emitters are applied to adisplay device, the display device will have a uniform brightness.

The photosensitive carbon nanotube slurry and cathode emitters madeusing the same have the following advantages. Firstly, thephotosensitive carbon nanotube slurry can form small-sized patternedcathode emitters (smaller than 100 microns) by the combination ofprinting and lithography, due to the existence of the photopolymerization monomer in the slurry. Secondly, the degradation of thefield emission performances of the cathode emitters can be effectivelyimpeded, due to the high electrical and thermal conductivity of thecarbon nanotubes.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A photosensitive carbon nanotube slurrycomprising a first mixture and a second mixture, wherein the firstmixture comprises carbon nanotubes, conducting particles and a firstorganic carrier; the second mixture comprises a photo polymerizationmonomer, a photo initiator and a second organic carrier; a weightpercentage of the first mixture in the photosensitive carbon nanotubeslurry ranges from about 50% to about 80%, and a weight percentage ofthe second mixture in the photosensitive carbon nanotube slurry rangesfrom about 20% to about 50%.
 2. The photosensitive carbon nanotubeslurry as claimed in claim 1, wherein the first mixture furthercomprises a binder and a stabilizer.
 3. The photosensitive carbonnanotube slurry as claimed in claim 2, wherein a weight percentage ofthe carbon nanotubes in the first mixture ranges from about 0.1% toabout 5%, a weight percentage of the conducting particles in the firstmixture ranges from about 20% to about 75%, a weight percentage of thebinder in the first mixture ranges from about 1% to about 25%, a weightpercentage of the stabilizer in the first mixture ranges from about 0.5%to about 5%, and a weight percentage of the first organic carrier in thefirst mixture ranges from about 20% to about 70%.
 4. The photosensitivecarbon nanotube slurry as claimed in claim 2, wherein a weightpercentage of the carbon nanotubes in the first mixture ranges fromabout 0.5% to about 2%, a weight percentage of the conducting particlesin the first mixture ranges from about 30% to about 70%, a weightpercentage of the binder in the first mixture ranges from about 5% toabout 15%, a weight percentage of the stabilizer in the first mixtureranges from about 1% to about 2%, and a weight percentage of the firstorganic carrier in the first mixture ranges from about 20% to about 55%.5. The photosensitive carbon nanotube slurry as claimed in claim 1,wherein the conducting particles is selected from the group consistingof silver particles, copper particles, gold particles, aluminumparticles and indium tin oxide particles.
 6. The photosensitive carbonnanotube slurry as claimed in claim 2, wherein the binder is selectedfrom the group consisting of glass powder, silicon oxide powder andsilicone resin powder, and a melting point of the binder is in a rangefrom about 350° C. to about 600° C.
 7. The photosensitive carbonnanotube slurry as claimed in claim 1, wherein the first organic carriercomprises ethyl cellulose, terpineol and ethanol, a weight percentage ofthe ethyl cellulose in the first organic carrier ranges from about 10%to about 40%, a weight percentage of the terpineol in the first organiccarrier ranges from about 30% to about 50%, and a weight percentage ofthe ethanol in the first organic carrier ranges from about 30% to about50%.
 8. The photosensitive carbon nanotube slurry as claimed in claim 1,wherein a weight percentage of the photo polymerization monomer in thesecond mixture ranges from about 20% to about 60%, a weight percentageof the photo initiator in the second mixture ranges from about 10% toabout 40%, and a weight percentage of the second organic carrier in thesecond mixture ranges from about 30% to about 40%.
 9. The photosensitivecarbon nanotube slurry as claimed in claim 1, wherein a weightpercentage of the photo polymerization monomer in the second mixtureranges from about 30% to about 50%, a weight percentage of the photoinitiator in the second mixture ranges from about 15% to about 35%, anda weight percentage of the second organic carrier in the second mixtureranges from about 30% to about 35%.
 10. The photosensitive carbonnanotube slurry as claimed in claim 1, wherein the photo polymerizationmonomer is selected from the group consisting of methyl acrylate, ethylacrylate, N-butyl acrylate, methyl methacrylate, ethyl methacrylate andN-butyl methacrylate.
 11. The photosensitive carbon nanotube slurry asclaimed in claim 1, wherein a particle size of the conducting particlesis less than or equal to 1 micron.
 12. The photosensitive carbonnanotube slurry as claimed in claim 1, wherein a specific surface areaof the conducting particles is in a range from about 1 m²/g to about 3m²/g.